1. Field of the Invention
This invention relates to the plasma vapor deposition of a layer of an electrically insulating material on a substrate by reactive sputtering. More particularly the inventive method and apparatus permits the sputter coating to be done using a DC plasma discharge.
2. Description of the Background Art
Sputter coating is a widely used technique for depositing a thin film of material on a substrate. In one form of this technique, known as DC sputtering, positive ions from a plasma discharge formed between an anode and a target cathode are attracted to and strike the target, dislodging the sputtering atoms from the target surface. Some of the dislodged atoms fall on the surface of the substrate and form a coating. In reactive sputtering a gaseous species is also present at the substrate surface and reacts with, and in some embodiments combines with, the atoms from the target surface to form the desired coating material. This material is also deposited on any other surface exposed to the sputtered atoms. It is recognized in the prior art that if the coating is an electrically insulating material, such as a metal oxide, the build up of the material on other parts of the sputtering apparatus can cause problems. In particular, the build up of an insulating coating on the anode interferes with the ability of the anode to remove electrons from the plasma, as required to maintain the plasma's charge balance. This destablizes the plasma and interferes with controlled deposition. As a result, it is common to use a different sputtering technique, RF sputtering, to deposit layers of insulating materials. However, RF sputtering is a less efficient, less controllable and more expensive process than DC sputtering.
Pinarbasi met this problem when trying to deposit layers of hydrogen-containing amorphous silicon. His work was published as a 1989 doctoral thesis for the University of Illinois at Urbana-Champaign entitled "Growth, Properties and Electrical Stability of DC Magnetron Reactive Sputtered Hydrogenated Amorphous Silicon Thin Films" and in Thin Films, 171 (1989 ) Pp. 217-233 . To reduce this effect, Pinarbasi shielded the anode with a positively biased anode shield to carry the electron current from the plasma. This reduced the problem enough to permit deposition of the experimental films. (See especially Page 26 and FIG. 7) But a poorly understood transient effect, reported by Pinarbasi, indicates that the problem was not completely under control. This problem could seriously impact long term use of this process, such as in a production setting.